Electric dynamic braking of alternating current (AC) induction and synchronous motors is one of several techniques used to reduce motor speed. During an electrical braking mode of operation, a motor acts as a generator to produce current and a corresponding retarding torque. In some systems, this regenerated current may be returned to the power source. When the power source is non-receptive to this regenerative current, the power may be dissipated in a dynamic braking resistance by conversion to thermal energy. Both regenerative and dynamic braking require current regulation in order to assure a smooth, constant torque braking action and to avoid motor instability.
In the past, dynamic braking control has been achieved by regulating the current magnitude through the braking resistance. In one form, as disclosed in U.S. Pat. No. 4,039,914, regulation has been accomplished by means of a chopper for generating variable width voltage pulses across the braking resistors. Typically, the same control circuit used for direct current (DC) rectification and chopping during the motoring mode operates during the braking mode to regulate current magnitude. A disadvantage of this control technique is that torque control necessary for smooth braking is limited by the available number of discrete resistance values which can be switched into the circuit in order to maintain steady current flow during braking since, as the motor slows, the regenerative power decreases.
One method which has been used to compensate for limited discrete resistance values employs high frequency switching regulators to modulate the resistance values. This method, while providing an effective means for smooth deceleration responsive changes in the resistive load, requires a high frequency switching circuit not otherwise needed in the motor propulsion system. It is desirable to achieve a smooth deceleration without the inclusion of such additional circuitry.